This invention relates to a method and apparatus for modifying a video signal so that a videotape recording of a video signal produces generally unacceptable pictures while a television monitor/receiver produces a generally normal picture from the modified signal. There exists a need for a method and system for modifying a video signal so that the signal produces a normal color picture on a television receiver, but videotape recording of the video signal is inhibited or prevented.
Some of the early work resulted in methods which are not satisfactory. For example, U.S. Pat. No. 4,163,253 was issued to Morio et al. in July of 1979. This patent describes an arrangement in which a positive pulse is added to the back porch portion of each horizontal blanking interval. This approach results in significant playability problems. In this connection, video cassette recorders designed for the consumer market place invariably feature some form of automatic gain control circuitry (AGC). The AGC circuit insures that the video level applied to the FM modulator in the recording system remains at a fixed, predetermined value, even if the video level applied to the machine's input terminal varies widely about the nominal value. Without an AGC system, high level signals would be distorted and clipped and low level signals would be marred by the noise and interference products generated by the recording process. If the input level dropped to less than 1/3+L of normal value, the replayed signal might not even be strong enough to reliably synchronize the receiver's timbales, hence giving rise to noisy unstable pictures. This early system, however, did not take into consideration the fact that many television monitor/receivers use the back portion region of a video signal for black-level clamping. The result is that while the approach described in the Morio et al. patent of confusing the AGC of a video cassette recorder does result in a copied tape being generally unviewable, the video signal also does not provide viewable pictures on many television monitor/receivers when it is used directly. This makes the process of limited value.
Another example of earlier work in the field of video copy protection is the work of Kagota in U.S. patent Ser. No. 4,475,129. This work generates a first pulse in the back porch of a horizontal blanking signal with a width narrower than the back porch and adding a second pulse substantially equal to the first pulse. These pulses are added to the back porch at predetermined time intervals. The approach used by Kagota has many of the same problems of Morio. The effectiveness of the copy protection does result in a copies tape being generally unviewable but the video signal does not provide viewable pictures on many television monitors/receivers when it is used directly. This make the process of low commercial value.
The instant inventor is responsible for more recent and more successful approaches to preventing the unauthorized recording of a video signal. Three of such approaches are described in U.S. Pat. Nos. 4,577,216, 4,631,603 and 4,819,098. Additionally the instant inventor is a co-inventor of U.S. Pat. No. 5,583,986. U.S. Pat. Nos. 4,577,216, 4,631,603, 4,819,098, 4,513,081 and 5,583,986 are incorporated herein by reference. The approach described and claimed in U.S. Pat. No. 4,631,603 and the approach described and claimed in U.S. Pat. No. 4,819,098 ('098) are similar to the Morio et al. arrangement, in that they rely on confusing the AGC of a video cassette recorder. They differ from the Morio et al. arrangement, however, in a major aspect. As in Morio, they rely on differences between a video cassette recorder and a television receiver to assure that while the modifications made to a video signal prevent copying by a video cassette recorder, they do not significantly affect the playability of such signal on a conventional television monitor/receiver. The '098 patent uses a different approach for modifying a composite video signal to confuse the AGC of a video cassette recorder as described above, while not materially affecting the playability of the resulting signal. That is, it has been found that by "clustering" during a field the modifications to a composite video signal responsible for confusing the AGC circuitry of a video cassette recorder, the effect of such modification on the playability of the video signal on a television receiver/monitor is significantly attenuated. Most desirably, the modifications of the signal take place during that portion thereof which is responsible for vertical blanking, i.e., the return of a beam sweep from the end of a raster scan field to the beginning. The video signal does not provide during such time a viewable portion of a video picture, with the result that any effect of the modifications to the video signal on the circuitry of a television receiver/monitor will not occur during a viewing interval. Thus, the deleterious affect on playability is virtually eliminated. While the '098 patent provides a very useful copy protection means, the "clustering" must be done carefully to balance between the effectiveness of the copy protection and the playability of the original signal.
The '810 patent has an embodiment, as described in FIG. 4 of the '810 patent, wherein the video level of the back porch area is lowered below the normal blanking level. This reduction has the affect of increasing the gain of the AGC system so as to cause the recorded picture to be brighter than normal and have a washed out look. This reduction of blanking level in the back porch area is not accompanied by an AGC pulse. There this lowering of the blanking level serves a different purpose than desired in the instant invention.
One of the elements described in the '936 patent includes the concept of horizontal sync narrowing so that when an illegal copy is made, the attenuated video with reduced sync pulse width (duration) causes a playability problem when viewed on a TV set. While this concept is useful in the instant invention, as will be seen below, the reason for reducing the sync width in the instant invention goes beyond improved effectiveness in the copy protection.